When have they done that? Most throttling over time is due to 2 reasons:

1. batteries degrading over time store less energy, so there’s less energy for the internal components. Workarounds can involve software throttling to avoid shutdown issues (which can corrupt your data) but the ultimate solution is a battery replacement. This isn’t inherent to software updates, it’s a consequence of a physical component breaking down over time.

2. engineers can choose to spend time maintaining compatibility with each new software release, or focus on optimizing for new hardware, or a little of both. Over time it’s possible later OS’ are less optimized than newer devices but this is not exclusive to Apple, and as many here have already pointed out Apple is way ahead of the competition in legacy device support already.

Can you prove that Apple has ever intentionally “slowed down older phones” in any other situation?

I’ve owned almost every iPhone since iPhone 4, multiple iPods before that, and have never had a lightning or 30-pin cable break down for me. Maybe yellowed a bit but never full on breaking like I’ve seen.

I suspect it’s because I’ve never allowed the cable to bend excessively from the port. I’ll bet everyone who complains about the cable longevity does the thing where the cable is cinched between the connector and whatever surface they’re propping their phone on. That causes cable strain and prematurely kills the cable.

In any case, your cable is covered by your phone’s limited warranty (or AppleCare), so as long as you have a device that takes that type of cable, Apple will replace for free.

Wasn't Apple accused of intentionally slowing down older phones as a tactic to get consumers to upgrade? IIRC they paid a huge settlement in court over it. Which I guess isn't necessarily an admission of guilt, but it doesnt look very good.